[Molecular genetics of inherited neuropathies]

Inherited neuropathies are clinically and genetically heterogeneous. At least 28 genes and 12 loci have been associated with Charcot-Marie-Tooth disease (CMT) and related inherited neuropathies. Most causes of inherited neuropathy have been discovered by positional cloning technique and in the past two years, the pace of CMT gene discovery has accelerated. Genetic studies have revealed the following gene mutations as the causes of inherited neuropathies; PMP22, MPZ, EGR2, SOX10, SIMPLE/LITAF, ARHGEF10 for CMT1 (autosomal dominant demyelinating form); GDAP1, MTMR2, SBF2/MTMR13, KIAA1985, NDRG1 PRX for CMT4 (autosomal recessive demyelinating form), MFN2, KIF1B, RAB7, GARS, NEFL, HSPB1, HSPB8 for CMT2 (autosomal dominant axonal form); LMNA, GAN1, KCC3, TDP1, APTX, SETX for AR-CMT2 (autosomal recessive axonal form); GIB1 for CMTX (X-linked CMT); DNM2 for CMT-DI (autosomal dominant CMT with intermediate nerve conduction velocities); and DHH for minifascicular neuropathy. These discovered CMT causing genes/proteins include those which show unpredictable correlations with the peripheral nervous system. However, these genes/proteins are definitely important for the peripheral nerve, and their discovery should pave the way for dramatic progress in the understanding of peripheral nerve biology. On the other hand, genotype-phenotype correlations of these genes are also important in order to understand the pathomechanisms of inherited neuropathy. Because, based on mutation studies, a large number of genes associated with both the CMT1/4 and CMT2 forms have been identified, it is usually difficult to predict the causative gene based on clinical information from patients without specific complications. To clarify the specific features and molecular mechanisms of five diseases that we previously reported, we reviewed recent progress in HMSN-P linked to chromosome 3, CMT4F caused by PRX, CMT4A caused by GDAP1, CMT4B2 caused by SBF2/MTMR13, and SCAN1 caused by TDP1. HMSN-P is characterized by late onset, proximal dominant severe muscle weakness, fasciculations, muscle cramp and sensory involvement. HMSN-P is a primary neuronopathy. Mutations in periaxin are associated with a broad spectrum of demyelinating neuropathies including DSS, a sensory dominant form and early onset slowly progressive CMT. Pathologically, loss of myelinated fibers, demyelination, small onion bulb formations, tomacula formation and myelin foldings were seen in sural nerves. Absence of septate like junction in the paranodal loop suggests that periaxin could be required for the adhesion complex. GDAP1 is a relatively common cause of CMT4. Half of reported patients showed the demyelinating form, while the rest showed the axonal form. The typical feature of CMT4A is paresis of the vocal cords and diaphragm. CMT4B2 is characterized by autosomal recessive, juvenile onset glaucoma and focally folded myelin in sural nerves. SBF2/MTMR13 mutations cause CMT4B2. Early onset glaucoma was seen in patients with nonsense mutations. SBF2/MTMR13 and MTMR2, which is the cause of CMT4B1, could be acting on the same 3-phosphoinositide signaling pathway. Clinical phenotypes of patients with TDP1, APTX, or SETX mutations share common clinical findings, namely cerebellar ataxia and axonal neuropathy. TDP1 and aprataxin both act on the single strand break repair pathway, with TDP1 working specifically on topoisomerase I related SSBR. Senataxin is a RNA helicase acting on RNA maturation and termination in yeast. Since these three proteins share a common pathway, disruption in any of them could induce a delay in the transcription process. The low rate of protein supply could lead to deaths of large neuronal cells.